Farrah Bashey

r- AND K-SELECTION REVISITED: THE ROLE OF POPULATION REGULATION IN LIFE-HISTORY EVOLUTION

The theory of r- and K-selection was one of the first predictive models for life-history evolution. It helped to galvanize the empirical field of comparative life-history and dominated thinking on the subject from the late 1960s through the 1970s. Large quan- tities of field data were collected that claimed to test predictions of the theory. By the early 1980s, sentiment about the theory had changed so completely that a proposal to test it or the use of it to interpret empirical results would likely be viewed as archaic and naive. The theory was displaced by demographic models that concentrated on mortality patterns as the cause of life-history evolution. Although demographic models are known for their density-independent approach and focus on extrinsic mortality, these models can incorporate many ecological features captured by r- and K-selection, such as density-dependent pop- ulation regulation, resource availability, and environmental fluctuations. We highlight the incorporation of these factors in recent theory, then show how they are manifest in our research on life-history evolution in Trinidadian guppies (Poecilia reticulata). Explanations of the repeatable suites of life-history differences across populations of guppies originate from demographic models of predator-driven age-specific mortality. Recently, careful ex- amination of guppy demography and habitat has revealed that density-dependent regulation and resource availability may have influenced the evolution of guppy life histories. In the field, these factors covary with predation risk; however, they can be uncoupled experi- mentally, providing insight into how they may have synergistically driven guppy life-history evolution. Although life-history theory has shifted away from a focus on r- and K-selection, the themes of density-dependent regulation, resource availability, and environmental fluc- tuations are integral to current demographic theory and are potentially important in any natural system.

CROSS-GENERATIONAL ENVIRONMENTAL EFFECTS AND THE EVOLUTION OF OFFSPRING SIZE IN THE TRINIDADIAN GUPPY POECILIA RETICULATA

Abstract The existence of adaptive phenotypic plasticity demands that we study the evolution of reaction norms, rather than just the evolution of fixed traits. This approach requires the examination of functional relationships among traits not only in a single environment but across environments and between traits and plasticity itself. In this study, I examined the interplay of plasticity and local adaptation of offspring size in the Trinidadian guppy, Poecilia reticulata. Guppies respond to food restriction by growing and reproducing less but also by producing larger offspring. This plastic difference in offspring size is of the same order of magnitude as evolved genetic differences among populations. Larger offspring sizes are thought to have evolved as an adaptation to the competitive environment faced by newborn guppies in some environments. If plastic responses to maternal food limitation can achieve the same fitness benefit, then why has guppy offspring size evolved at all? To explore this question, I examined the plastic response to food level of females from two natural populations that experience different selective environments. My goals were to examine whether the plastic responses to food level varied between populations, test the consequences of maternal manipulation of offspring size for offspring fitness, and assess whether costs of plasticity exist that could account for the evolution of mean offspring size across populations. In each population, full‐sib sisters were exposed to either a low‐ or high‐food treatment. Females from both populations produced larger, leaner offspring in response to food limitation. However, the population that was thought to have a history of selection for larger offspring was less plastic in its investment per offspring in response to maternal mass, maternal food level, and fecundity than the population under selection for small offspring size. To test the consequences of maternal manipulation of offspring size for offspring fitness, I raised the offspring of low‐ and high‐food mothers in either low‐ or high‐food environments. No maternal effects were detected at high food levels, supporting the prediction that mothers should increase fecundity rather than offspring size in noncompetitive environments. For offspring raised under low food levels, maternal effects on juvenile size and male size at maturity varied significantly between populations, reflecting their initial differences in maternal manipulation of offspring size; nevertheless, in both populations, increased investment per offspring increased offspring fitness. Several correlates of plasticity in investment per offspring that could affect the evolution of offspring size in guppies were identified. Under low‐food conditions, mothers from more plastic families invested more in future reproduction and less in their own soma. Similarly, offspring from more plastic families were smaller as juveniles and female offspring reproduced earlier. These correlations suggest that a fixed, high level of investment per offspring might be favored over a plastic response in a chronically low‐resource environment or in an environment that selects for lower reproductive effort

A Comparison of the Suitability of Alizarin Red S and Calcein for Inducing a Nonlethally Detectable Mark in Juvenile Guppies

Abstract Mark–recapture studies are an important component of fisheries research. A diversity of marks is needed to meet the demands of experimental designs and to overcome species-specific variation in marking success. Suitable marks must not alter the viability of marked individuals, must be easy to detect, and must be retained for an appropriate period of time. I compared the effect of alizarin red S and calcein on the individual growth and mortality rates of guppies Poecilia reticulata via short-term experiments (<14 d) conducted both in environments where alizarin- and calcein-marked fish were allowed to interact with unmarked fish and in environments where fish were segregated by mark. Neither mark affected the growth or mortality of marked individuals. Both marks were easily applied, did not affect the appearance of fish, and could be detected on the skeleton of live, anesthetized fish or ethanol-preserved specimens without the additional preparation (or lethality) involved in detecting marks on ot...

The Suitability of Calcein to Mark Poeciliid Fish and a New Method of Detection

Abstract The suitability of calcein as a marker for population studies depends on (1) the assumption that marked individuals have unaltered viability, (2) the fidelity of the calcein label, and (3) the facility with which calcein can be used. We examined the effects of calcein on survival, growth, and the timing and size at sexual maturity of least killifish Heterandria formosa and present a new method for detecting calcein. To test the assumption that marked individuals have unaltered viability, juvenile least killifish were immersed for 24 h in a 250-mg/L solution of calcein. A control group of same-aged juveniles was immersed in the same volume of water for 24 h without calcein. All individuals were then removed and reared individually in separate containers. Upon examination under an epifluorescent microscope, all calcein-treated fish showed fluorescent green marks on their scales and fin rays, whereas controls did not. The calcein treatment had no significant effect on growth and survival through 9 w...

THE EVOLUTION OF SPITE: POPULATION STRUCTURE AND BACTERIOCIN‐MEDIATED ANTAGONISM IN TWO NATURAL POPULATIONS OF XENORHABDUS BACTERIA

Spite occurs when an individual harms itself in the act of harming other individuals. Such behaviors were once assumed to be of limited evolutionary importance, as the conditions for the evolution of spite were thought to be too restrictive. Recent theoretical work, however, suggests that spatial population structure, which allows local competition among genotypes, could favor the evolution of spite. One of the clearest examples of spite is the costly production and release by bacteria of toxins (called bacteriocins) that can kill unrelated strains of the same species. Here, we establish the existence of spatial structure in two natural populations of bacteriocin‐producing bacteria. Specifically, relatedness decreased with increasing spatial distance between the field isolates. In addition, toxin‐mediated inhibitions were found only between isolates that were collected more than 1 m apart and that were generally less than 80% similar in their genomic fingerprints. Taken together, the results suggest that the bacteria are spatially structured, with mixing of genotypes and spiteful interactions at the boundaries between demes.

Bacteriocin-mediated interactions within and between coexisting species

Bacteriocins are bacteriocidal toxins released by almost all bacteria. They are thought to have a narrow range of killing, but as bacteriocin-mediated interactions have been rarely studied at biologically relevant scales, whether this narrow range of action falls mostly within or mostly between coexisting species in natural communities is an open question with important ecological and evolutionary implications. In a previous study, we systematically sampled Xenorhabdus bacteria along a hillside and found evidence for genotypic variability and bacteriocin-mediated interactions within Xenorhabdus bovienii and X. koppenhoeferi colonies that were collected only a few meters apart. In contrast, colonies that were isolated from the same soil sample were always genetically similar and showed no inhibitions. Here, we conducted pairwise growth-inhibition assays within and between seven X. bovienii and five X. koppenhoeferi colonies that were isolated from different soil samples; all seven X. bovienii colonies and at least three of the X. koppenhoeferi have been distinguished as distinct genotypes based on coarse-grain genomic markers. We found signatures for both conspecific and heterospecific bacteriocin inhibitions in this natural community of Xenorhabdus bacteria, but intraspecific inhibitions were significantly more common than interspecific inhibitions. These results suggest that bacteriocins have a major role in intraspecific competition in nature, but also suggest that bacterocins are important in mediating interspecific interactions among coexisting species in natural communities.

Low migration decreases interference competition among parasites and increases virulence

Competition among different parasite genotypes within a host is predicted to affect virulence. The direction of this effect, however, depends critically on the mechanisms that parasites use to compete or to cooperate with each other. One mechanism that bacteria use to compete with each other is via the production of bacteria‐killing toxins, called bacteriocins. This warfare among parasites within a host is predicted to reduce the rate of host exploitation, resulting in lower virulence. By contrast, if parasites within a host are highly related, there could be a reduction in within‐host conflict, increasing virulence. We examined this idea by allowing an insect‐parasitic nematode (Steinernema carpocapsae) and its symbiotic bacteria (Xenorhabdus nematophila) to evolve for 20 passages under two different migration treatments (low and high). We found that host mortality rates were higher in the low‐migration treatment when compared with the high‐migration treatment. In addition, bacteria isolated from the same insect host inhibited each other’s growth, but only in the high‐migration treatment. These results show that population structure and interactions among parasites within hosts can be critical to understanding virulence.

Spiteful Interactions in a natural population of the bacterium Xenorhabdus bovienii.

An individual behaves spitefully when it harms itself in the act of harming other individuals. One of the clearest potential examples of spite is the costly production and release of toxins called bacteriocins. Bacteriocins are toxins produced by bacteria that can kill closely related strains of the same species. Theoretical work has predicted that bacteriocin‐mediated interactions could play an important role in maintaining local genetic and/or species diversity, but these interactions have not been studied at biologically relevant scales in nature. Here we studied toxin production and among‐strain inhibitions in a natural population of Xenorhabdus bovienii. We found genetic differences and inhibitions between colonies that were collected only a few meters apart. These results suggest that spite exists in natural populations of bacteria.

Spiteful interactions between sympatric natural isolates of Xenorhabdus bovienii benefit kin and reduce virulence

Spite occurs when an individual harms itself in the act of harming others. Spiteful behaviour may be more pervasive in nature than commonly thought. One of the clearest examples of spite is the costly production and release of bacteriocins, antimicrobial toxins noted for their ability to kill conspecifics. A key question is to what extent these toxins provide a fitness advantage to kin of the producer cell, especially in natural communities. Additionally, when bacteria are involved in parasitic relationships, spiteful interactions are predicted to lower bacterial densities within a host, causing a reduction in parasite‐induced virulence. Using five sympatric, field‐collected genotypes of the insect pathogen Xenorhabdus bovienii, we experimentally demonstrate that bacteriocin production benefits kin within the host, and that it slows the mortality rate of the host. These results confirm that spite among naturally coexisting bacterial clones can be a successful kin‐selected strategy that has emergent effects on virulence.

ALTERNATIVE PATHS TO SUCCESS IN A PARASITE COMMUNITY: WITHIN-HOST COMPETITION CAN FAVOR HIGHER VIRULENCE OR DIRECT INTERFERENCE

Selection imposed by coinfection may vary with the mechanism of within‐host competition between parasites. Exploitative competition is predicted to favor more virulent parasites, whereas interference competition may result in lower virulence. Here, we examine whether exploitative or interference competition determines the outcome of competition between two nematode species (Steinernema spp.), which in combination with their bacterial symbionts (Xenorhabdus spp.), infect and kill insect hosts. Multiple isolates of each nematode species, carrying their naturally associated bacteria, were characterized by (1) the rate at which they killed insect hosts, and by (2) the ability of their bacteria to interfere with each others growth via bacteriocidal toxins called “bacteriocins.” We found that both exploitative and interference abilities were important in predicting which species had a selective advantage in pairwise competition experiments. When nematodes carried bacteria that did not interact via bacteriocins, the faster killing isolate had a competitive advantage. Alternatively, nematodes could gain a competitive advantage when they carried bacteria able to inhibit the bacteria of their competitor. Thus, the combination of nematode/bacterial traits that led to competitive success depended on which isolates were paired, suggesting that variation in competitive interactions may be important for maintaining species diversity in this community.